1. Field of the Invention
This invention generally relates to electric-field manipulation of ink drops in printing. In particular, the invention relates to electric-field acceleration and steering of the ink drops to increase the placement accuracy of the ink drops and the resolution capability of a printer.
2. Description of Related Art
Conventional ink drop printing systems use various different methods to produce ink drops directed toward a print substrate. Well-known devices for ink drop printing include thermal ink jet printheads, piezoelectric transducer-type ink jet printheads, and acoustic ink jet printheads. All of these technologies produce roughly spherical ink drops having a 15-100 micron diameter directed toward a print substrate at approximately 4 m/sec. The actuators in the printheads which produce the ink drops are controlled by a printer controller. The printer controller activates the actuators in conjunction with movement of the print substrate relative to the printhead. By controlling the activation of the actuators and the print substrate movement, the print controller directs the ink drops to impact the print substrate in a specific pattern, thus forming an image on the print substrate.
Ideally, all of the actuators in a printhead produce ink drops directed toward the print substrate in a direction perpendicular to the print substrate. In practice, however, some ink drops are not directed exactly perpendicular to the print substrate. The ink drops which deviate from the desired trajectory are undesirable since the misdirected drops impact the print substrate at a point not anticipated by the print controller. Therefore, misdirected drops affect the quality of the printed image by impacting the print substrate in unwanted positions.
U.S. Pat. Nos. 4,386,358 and 4,379,301 to Fischbeck disclose a method for electrostatically deflecting electrically charged ink drops ejected from an ink jet printhead. Charges placed on electrodes on the printhead disclosed by Fischbeck are controlled to steer the charged ink drops in desired directions to compensate for known printhead movement. By electrostatically steering the charged ink drops, the method disclosed in Fischbeck compensates for ink drop misdirection caused by the known printhead movement when the ink drop is ejected.
However, the electrostatic deflection method disclosed by Fischbeck does not compensate for unpredictable environmental factors which can affect ink drop trajectories. Such environmental factors include air currents and temperature gradients between the printhead and the print substrate. In acoustic ink jet printheads, unpredictable variations in the dynamics of ink drop creation also detrimentally affect ink drop trajectories. Some of the variations in ink drop creation are caused by aberrations in the lithography of the Fresnel lens which focusses the acoustic wave used to create the ink drops.